Method of and means for magnetic inspection



Nov. 15, 1938. v, 5 FOREST 2,136,375

METHOD OF AND MEANS FOR MAGNETIC INSPECTION Filed Aug. 22, 1956 2Sheets-Sheet 1 INVENTOR 41mm Mia/0x55 T g BY - W, ATTORNEY Nov. 15,1938. A. v. DE FOREST 2,136,375

METHOD OF AND MEANS FOR MAGNETIC INSPECTION Filed Aug. 22, 1936 2Sheets-Sheet 2 R BY 1 W, ATTORNEY Patented Nov. 15,1938

PATENT OFFICE METHOD OF AND MEANS FOR MAGNETIC INSPECTION Alfred V. dcForest, Cambridge, Mass, assignor to Magnailux Corporation, Chicago,11]., a corporation of Delaware Application August 22, 1938, Serial No.97,415

32 Claims. (Cl. 175-183) The present invention relates to improvementsin methods of and means for inspecting magnetizable materials in whichfinely divided ferro-magnetic particles are employed to reveal thepresence of minute cracks or other defects. These methods involve thecreation of a magnetic field at the edges of the crack or other detectwhich may be accomplished by placing the specimen under test on thepolesof a magnetic yoke or electro-magnet, the latter being energized bydirect current. This method is disclosed in Patent No. 1,426,384, issuedto William E. Hoke, August 22, 1922.

It is also possible to create the desired magnetic field by passing aheavy current through the specimen which induces a magnetic field atright angles to the direction of current flow. The current may be eitherdirect or alternating as described in my Patent No. 1,960,898, issuedMay 29, 1934.

In carrying out the method disclosed in Patent No. 1,960,898, it isnecessary to establish good electrical contact with the specimen whichwill carry several thousand amperes per square inch. This is customarilydone by clamping the specimen in a machine built for the purpose withspecially shaped copper contact blocks or with flexible copper braidbetween the specimen and the conductor. Even with all these precautionsthere is the danger of burning a test specimen at the contact point ormarring a finished surface, such, for instance, as that of a bearingball.

It is an object of the present invention to overcome this diiliculty bysupplying the magnetizing current for such a short duration of time thatsuflicient heat will not be developed to burn the surface. At the sametime the voltage is greatly increased so that contact resistance is nolonger a limitation on the current flow. It has been the usual practicewhen inspecting such parts as crank shafts to employ about 2000 amperesD. C. or- 1000 amperes, 60 cycle A. 0., per square inch of the sectionalarea under test, at a potential of between 4 and 16 volts, dependingupon the equipment. In contrast to this method, the present methodconsists in carrying a current through the part of roughly the samevalue as before, that is, about 1000 to 2000 amperes, but for only aminute part of a second, say, one ten thousandth or one one-hundredthousandth part of a second, or even less. At the same time thepotential may be increased to between 200 and 2000 volts or more,without diiiiculty. It is obvious that the heating eilect is directlyproportional to the amount and duration of electrical current applied,while the magnetization is directly proportional to the amperage. Thus,by reducing the time factor to a minimum the heating effect iscorrespondingly reduced without reducing the magnetizing effect, exceptthrough the factor known as skin effect" which limits only the depth towhich magnetization penetrates.

Another object of my invention is to provide a method which can becarried out by much lighter equipment than has heretofore been possible.I have found that by the use of electrical impulses of very shortduration, the flow, of current and the magnetization, as well, arerestricted to the extreme surface of the specimen so that the requiredcurrent to magnetize the specimen is a function of its area rather thanits volume. The small amount of energy necessary to provide magnetizingcurrent for such a short time can readily be taken from a condenser.Obviously, this permits of the use of lighter equipment.

A further object of my inventionis to provide means for producing a highfrequency oscillation and suppressing all but single waves atpredetermined time intervals so that only isolated impulses areutilized.

A further object of the invention is to provide means whereby a magneticfield may be swept over a surface in such direction as to causeprogressive magnetization thereof, each impulse of current producingmagnetization in a different zone of the surface.

Other objects and advantages of my invention will appear in thefollowing description of a thereof and also of certain means forcarrying out such methods.

In the accompanying drawings;

Figure 1 is a diagrammatic showing of the ordinary ,wave form ofenergizing current in which the negative impulses are eliminated;

Fig. 2 is a diagrammatic showing of my improved form of energizingcurrent comprising widely spaced unidirectional impulses;

Fig. 3 shows diagrammatically a set of widely spaced impulses ofalternately opposite polary;

Fig. 4 is a diagram of electrical connections illustrating an apparatusfor producing spaced unidirectional impulses of the type shown in Fig.2;

Fig. 5 is a diagram of electrical connections in an apparatus similar tothat shown in Fig. 4,

'preferred method and certain modifications widely spaced revealingthem.

but with a relay control of the impulse producing apparatus;

Fig. 6 is a modification of the apparatus in the snown which impulsesare usiE alternating current supparatus;

.lect; cal diagram of a mechanirec: currerr energised sysproducingmoving contact; rspective a testing device moving f turbine .d Q 2cracks that r wise, of an tanecusly detects Fig. is e specimen may altfor producing nately polarity such as 13 shows netizing surro' 14 showsin p rspectiv a means for ap= plying my method to an annular speci men,such, for instance, as a ball race ring, where the desired magnetizationis circular.

My method of inspection involves first the creation of a magnetic fieldin the specimen with the lines of force disposed transversely to thesupposed direction the cracks or other defects. If the specimer composedof material that will retain magneti it may be magnetized by passingalternating current therethrough, and in order to leave residualmagnetic field in the specimen it is necessary to interrupt the currentafter it has reached maximum value and before has passed the zero pointand has started in the opposite direction to cause demagnetization;after the specimen has been magnetized fine particles of term-magneticmaterial may be applied thereto, such, for instance, as

described in my Patent No. 1,960,899, and the particles will clusterabout the cracks, prominently if the specimen under test is of amaterial which will not retain magnetism, such, for instance, as wroughtiron or low carbon steel, the magnetic testing powder may be applied tothe specimen and then be slightly tapped or vibrated while the currentis being passed through the specimen so that the particles will beassisted in moving to the points where the magnetic field is disturbedby cracks or other defects.

One of the great advantages of my method is the fact that contactresistance may be discounted by the employment of high voltage. Thispermits of the use of moving contacts whereby a broad area may beinspected by progressive magnetization. For instance, in testing aturbine blade which is apt to develop slight defects or cracks runningcrosswise oi the blade my method permits of applying contact brushes ofan energizing circuit to opposite edges of the blade and moving thesebrushes along the entire length of the blade. The current pursues adirect course from one brush to the other across the blade but since thebrushes are moved lengthwise of the blade the entire area. willeventually have been subjected to the energizing current.

. eliminating such portions of This, according to the well known laws ofphysics, develops a magnetic field running at right angles the directionof the current and this magnetic field intercepts the cracks that areapt to develop a turbine blade.

in order to produce progressive magnetization of the blade it isnecessary to employ only unidirectional impulses. For instance, inFigure l, a sine wave, a, b, c, e, is shown in which the negativeportion 0, 5, shown in broken lines, has been eliminated, leaving onl;the positive portion a, b, o. By thus rectifying the current, a seriesof unidirectional impulses will he passed through the specimen,progressively magnetizing the specimen.

The wave form shown in Fig. 1 r presents that of commercial alternatingcurrent of say i) cycles per second which has been merely rectified byeach wave ass below the zero line a y. heat such waves, however, is highbecause of toe time during which current applied ,4 sec. for eachimpulse). In order to reduce this heating effect, provide means forconverting a low cycle current into high frequency cscii ations andsuppress not merely such portions of each oscillotion as pass below thezero line and also sup press all but single half wave oscillations whichtake place at fixed intervals. Thus, I am able to produce a wave formcorresponding to that shown in Fig. 2. This wave form will have the sameamplitude as that shown in Fig. l and the intervals between impulses maybe the same. but the length of time the current is applied and thereforethe heating effect of each impulse will be a very small fraction of thatdeveloped by the wave form shown in Fig. 1. For instance, I may convert60 cycle frequency into 100,000 cycle frequency but utilize only oneunidirectional impulse at each one-sixtieth of a second, suppressing allthe intervening wave impulses. Although the heat energy developed bysuch relatively widely spaced impulses is slight, yet the surfacemagnetizing eflect will be fully as great as that of the wave form shownin Fig. 1. With such relatively occasional impulses it is possible touse a high voltage with no substantial loss at the contacts.

I show diagrammatically in Fig. 4 a preferred apparatus for producingwidely spaced high frequency unidirectional impulses. In this apparatusan electronic tube i5 is employed to control the periodic discharge 01'a condenser 16. specimen under inspection is indicated at I! and isengaged by a pair of brushes I9 and 20. The brush 20 is connected byline 2| to one side of the condenser i6, while the other brush I 9 isconnected by line 22 to the cathode 23 of the gas filled tube IS. Theanode 24 of said tube is con nected by a line 25 to the other side ofthe condenser IG. Within the tube I5 are the grids 21 and 28. The grid21 is connected through a reslstance 29 to a variable point on aresistance 30 bridging the lines 25 and 22. The grid 28 is connectedthrough a. resistance iii to the line 25 and through a resistance 32 tothe line 22.

The condenser i5 is arranged to be charged through a current limitingresistor 34 from a conventional direct current power supply of say 400volts. If desired, commercial direct current may be employed, or,alternating current may be used provided the current is rectified. InFig. 4, I show a conventional means for rectifying alternating current,such means comprising a transformer 35 and a rectifying tube 36. Thesupareas-rs ply of current is controlled by a switch 51 which ispreferably closed only after the brushes l5 and 20 have made propercontact with the specimen 48.

In operation, when the voltage across the condenser l6 reaches apredetermined value, say 350 volts, the tube, i5 becomes conducting andpermits the condenser to discharge through it and through the specimen.Since the impedance of the discharge path is low the current can rise tovery high values-500 to 2,000 amperes. Whenthe condenser I 6 iscompletely discharged the tube l5 becomes non-conducting and allows thecondenser to recharge through the resistance 34. The critical voltage atwhich the tube becomes conducting depends upon the bias voltages on itsgrids which voltages are determined by the bias resistors 30, 3| and 32.The circuit is of the well-known relaxation oscillator type and willgive current impulses at a rate determined by the supply voltage, thecondenser capacity, and the break-down voltage of the tube l5. It willbe understood that in order to handle large currents the tube I5 must beof a special type, as most gas filled tubes, particularly those with hotcathodes will not stand such large currents as I employ in my inspectionsystem. The current impulses are unidirectional because the tube 15prevents the current in the discharge circuit from reversing due to theoscillatory nature of the circuit.

In practice it is desirable to prevent the brushes i 9 and 20 frompassing current until after a good contact is'made with the work. Tothis end, a low voltage relay circuit may be used such as illustrated inFig. 5. The relay circuit 38, which bridges the brushes I9 and 20, isenergized by a battery 39. In this circuit there is a relay 40 whichwhen energized closes a gap 4| in the line 25. However, the relay willnot be energized sufficiently to close the gap until the brushes havemade good contact with the work. If desired a push button or othercontrol switch 42 may be provided in the relay circuit but in any casethe operation of the impulse generating apparatus will depend upon theestablishment of proper electrical contact between the brushes and thework. This will prevent the operator from receiving an accidental shock.

It is possible to make the circuit operate in cynchronism with thealternating current line voltage by applying voltage directly from thetransformer to one of the grids of the tube through resistance 3i, asshown in Fig. 6. In this figure parts which correspond to those in Fig.4 are given the same reference numeral. It will be observed, however,that the resistor 3| in Fig. 6 is not connected with the line 25, but isconnected by a line 43 to the secondary coil of the transformer 35.Thus, if the line current alternates at 60 cyiles per second, theimpulses passing through the specimen l8 will also occur at the rate of60 per second, but the impulses will still be isolated and relativelywidely spaced from each other.

In Fig. 7, I show a mechanical means for producing electrical impulsesof short duration with direct current. In this case, a source of directcurrent is supplied across the liner-1.45 and 4'5. In the line 45 thereis a resistance 41 and a mechanical switch 48 which connects itperiodically to the brush or contact 49 hearing on one side of thespecimen 50. The other line 46 extends to the opposite brush 5!. Acondenser 52 is bridged across lines 45 and 46 and between thiscondenser and the brush 5| there is a resistance 53. The switch 45 ismechanically rotated at suitable speed and each time theswitch makescontact there will be a discharge of current stored in the condenser 52which will produce a momentary impulse running through the specimen 5..Oi

dinarily, such a discharge would result in a series of rapidly dampedoscillations. However, by introducing the resistance 53 the damping maybe increased to such an extent as to produce substantially a singleunidirectional impulse at each closure of the switch. It will beunderstood that there are other ways in which widely spaced impulses,such as indicated in Fig. 2, may be obtained. A

In Fig. 8, I illustrate an application of my method to the testing of ablade-like specimen 54 which is composedof a metal that will retainmagnetism; The specimen is attached at one end to a shaft 55 which isconnected to a conductor 55 by means of a fixed brush or electricalcontact 51. A brush 58 is connected to a conductor 59 which forms theopposite pole of the impulse producing apparatus. This brush may beconveniently made of copper gauze or other suitable material which willgive a broad and flexible contacting surface. In operation, the circuit,represented fragmentally by the conductors 55 and 59. is energized withperiodic unidirectional impulses and the brush 58, at the same time, ismoved along the edge of the b'ade 54 toward the shaft 55., The currentimpulses will flow from brush 58 to shaft 55 and thence to line 55, andin doing so will enter the edge of the specimen following in general thedotted lines shown in Fig. 8. In other words, the current will enter theblade at right angles to the edge and then will curve toward theshaft'55. The brush 58 will be moved gradually along the edge of theblade toward the shaft and each impulse of electric current will producea corresponding magnetic field which will be retained in the blade 54,and the direction of this field will be substantially parallel to theedge of the blade along which the brush is moved.

Thissystem oi. testing is particularly applicable to inspection ofturbine blades which are apt to develop transversecracks at the leadingedge of the blade.' Such cracks would be disposed at right angles to thedirection of the magnetic field created by the electric flow. Thepenetration of this field will be confined rather closely to the 'edgeof the blade and the test is applied to the leading edge of the blade.If it be desired to test the opposite edge, as well, the contact brush58 may also be moved along the opposite edge of the blade.

Instead of having one fixed contact and one moving brush two opposedbrushes may be employed, as shown particularly in Fig. 9. In thisfigure, I show a pair of contact arms 60 and 5| which are hingedtogether intermediate their ends, as indicated at, 62. The arms areprovided with contact brushes 63 and 64 respectively, and extensions ofthese arms at the opposite side of the pivot 62 form handles 65 and 66,respectively. These handles are normally pressed apart by a spring 61which causes the brushes B3 and 84 to engage opposite sides of aspecimen 68 to be tested. brushes 63 and 64 through leads 58 and II. Thebrushes are suitably insulated from the arms 50 and 6| by blocks ofinsulation material indicate at H and 12, respectively.

In operation, the brushes are applied to oppo- Testing current issupplied to the themagnetic lines of force.

- specimens at a time.

. surface.

site edges of the specimen 88 which is to be tested, the handles 65 and88 being squeezed to spread apart the brushes, and then on releasing thehandles the spring 61 exerts sufficient pressure to maintain the desiredcontact of the brushes with the edges of the specimen 88. The current isapplied and at the same time the device is drawn lengthwise of thespecimen so that successive electrical impulses pass through parallelpaths transversely with respect to the longitudinal axis of thespecimen. This results in the production of successive magnetic fieldsextending lengthwise of the specimen so that any cracks orinhomogenities, such as indicated at 13, will interrupt Magnetic testingpowder is then applied to the specimen and the powder will collect atpoints where the magnetic lines are interrupted, indicating the defects.

If desired, the test may be applied to several For instance, in Fig. 10,I show a pair of turbine blades 15 and 18 being simultaneouslymagnetized by means of a device similar to that shown in Fig. 9, butdiffering therefrom in having contact blocks 88a and 64a which are broadenough to bridge the pair of turbine blades.

While Figs..8 to 10 show moving contact methods applied to blade-likeforms the same systems may be used to magnetize portions of large piecesof material of any form. For instance, in searching for forging cracksor flakes in sections of billets, two brushes set at a distance apartor" three to six inches may he so moved as to magnetize successively allportions of the On large shafts of machines where cracks are suspectedat fillets and keyways, these locations can be explored by drawing thebrush contacts over the part with the current between contacts roughlyparallel to the probable direction of the crack.

Fig. 11 shows another application of my improved method in which it isdesired to detect cracks that extend longitudinally, as well astransversely, of a specimen 17. In this case, contact blocks 83b and 64bare provided which are not disposed directly opposite each other but areoff-set lengthwise of the specimen, so that the how of current from onecontact block to the other will be oblique with respect to thelongitudinal axis of the specimen. Such current will set up a magneticfield at right angles to the paths pursued by the electrical impulsesand such magnetic fields when the magnetic testing powder is appliedthereto, will betray the presence of all cracks, except those which aredisposed parallel to the field.

In all cases where movable contacts are employed it is desirable to makegood contact with the brushes before the current is turned on in thecircuit, and to turn off this current before the contact is broken. Thismay be done b; employing the system shown in Fig. 5 and the push button42 may be conveniently located, as, for instance, on the handle 65 ofthe tool shown in Fig. 9.

While it is necessary to use unidirectional impulses in the case ofsliding contacts it will be evident that where the contacts are fixed,alternating current impulses may be employed, such as shown in Fig. 3. Acircuit for producing such impulses of alternately opposite polarity isshown in Fig. 12. In this circuit there are two stroboscopic tubes Bi!and 8| connected in parallel and fed with direct or rectified current ofsuitable potential, say 400 volts. The specimen 82 under .to the brush83 and the other side thereof is connected through tube 8| to brush 84,while one side of condenser 86 is connected to the brush 84 and theother through tube to brush 8!. Thus,-the path of discharge through thespecimen will be in one direction for condenser 85 and in the oppositedirection for condenser 88, as shown by the broken lines 85a and 86a.The tube which reaches a critical potential first will fire, dischargingits condenser. Since the tubes are never exactly alike they will neverfire at the same time. The bias on the tubes is adjusted so that theytend to run at the same frequency. When acondenser is discharged thevoltage across .it is low. This will increase the voltage drop in itsresistor and will slightly reduce the voltage on the other condenserwhich will tend to prevent the other tube from firing until the firstcondenser has had a chance to partly recharge. This effect will causethe tubes to fire alternately and thus produce current pulses throughthe specimen, such as shown in Fig. 3.

While the method as so far described has called for the passage of thecurrent through the speci men, it will be obvious that the same effectcould be obtained by placing the specimen within a coil through whichthe impulses pass and thereby inducing electrical impulses in thespecimen. Fig. l3,'for instance, shows a winding 90 of solenoid formsuitable for longitudinal magnetization of I bar stock 9! or similarshapes by impulses of high current for short lengths of time. Thewinding must be properly proportioned for the rapid dis-- charge ofcurrent.

In Fig. 14, a ball race ring is shown within a winding 95 through whichimpulses of e1ectrical current are passed. If the impulses are of highamplitude but of relatively wide spacing and small area, such as shownin Fig. 3, the heat developed is comparatively small and yet a highmagnetization is obtained. After the ball race has been magnetized, itis subjected to test with the magnetic testing powder to reveal suchflaws as will show up as the powder clusters about the cracks or otherdefects in the specimen.

While I have shown a number of systems for producing isolated electricimpulses of high intensity and potential but individually of very minuteduration, I do not wish to be limited to the particular systemsdescribed. It will be understood therefore that I reserve the right toemploy other means for producing such impulses and for applying suchimpulses to specimens under test without departing from the spirit andscope of my invention as set forth in the claims.

I claim:

1. The method of magnetizing a ferro-magnetic object for the purpose oflocating a supposed defect therein, which consists in passing an impulseof electric current through the object in such direction as to induce amagnetic field disposed transversely to the supposed direction of saiddefect, said impulse being of high voltage and amperage but of minuteduration not exceeding one one-thousandth of a second.

2. The method of magnetizing a ferro-magnetic object for the purpose oflocating a supposed defect. therein, which consists in passing aunidirectional impulse of electric current through the object in suchdirection as to induce a magnetic viii) field-disposed-transversely tothe supposed direction of said defect, said impulse being of highvoltage and amperage but of minute duration not exceeding oneone-thousandth of a second.

3. The method of magnetizing a ferro-magnetic object for the purpose oflocating supposed defects therein, which consists in subjecting theobiect to periodic pulsations of electricity of high voltage andamperage but individually of small duration and separated byinert'intervals, and so directing said pulsations as to induce amagnetic field in the object disposed transversely to the supposeddirection of said defects.

4. The method of magnetizing a ferro-magnetic object for the purpose oflocating supposed defects therein, which consists in producing highfrequency oscillations of electric current of high voltage and amperage,suppressing all but isolated oscillations at comparatively widely spacedintervals, passing said isolated impulses through the object in suchdirection as to induce a magnetic field in the object of a directiontransverse to the supposed direction of said defects.

5.-The method of magnetizing a form-magnetic object for the purpose oflocating supposed der-fects therein, which consists in subjecting theobject to periodic unidirectional electric pulsations of high voltageand amperage but individually of small duration and separated by inertintervals, so directing the pulsations as to induce a magnetic field inthe object of a direction transverse to the supposed direction of saiddefect, andcansing displacement of the object withrespect to thepulsations in a direction transverse to the direction of the pulsations.

6. The method of magnetizing a ferro-magnetic object for the purpose oflocating supposed-defects therein, which consists in generating isolatedunidirectional electric impulses of high 'voltage and amperage butindividually of minute duration and separated from each other by inertperiods, each inert period being many times greater than the duration ofeach impulse, passing said impulses through the object in such directionas to induce a magnetic field of a direction transverse to the supposeddirection of said defects and causing a lateral displacement ofsuccessive impulses whereby the energizing eflect of the impulses willbe spread over a broad area of the object.

7. The method of exploring a farm-magnetic object for supposed defectstherein, which consists in inducing a magnetic field in the object in adirection transverse to the supposed direction of said defects bypassing isolated pulsations of electric current through the object atright angles to the desired direction of said field, eachpulsation beingof high voltage and amperage but of minute duration and spaced from thenext pulsation by an inert interval greater than the period of durationof the pulsation, and applying finely divided 'ferro-magnetic particlesto the ob- .ject, whereby the particles will cluster about any defectstraversed by the magnetic field and will thereby reveal their presence.

8. The method of magnetizing an object'of ferro-magnetic material forthe purpose of loeating supposed defects therein, which consists ingenerating isolated unidirectional impulses of high voltage and amperagebutindividually of minute duration and spaced apart by comparativelylong inert periods, passing-said .impulses through the object between afixed point on the object and an-opposed point-movable along the objectat an angle to a direct line connecting said ,end thereof and a contactpoint on said edge, and

moving the contact point along said edge.

10. The method oi magnetizing an elongated object of term-magneticmaterial for the purpose of locating supposed defects therein, saiddefects being supposedly directed transversely to the longitudinalaxis'of the object, which consists in generating isolated unidirectionalelectric impulses of high voltage and amperage but individually ofminuteduration and relatively widely spaced apart, passing said impulsesthrough the object at right angles to the longitudinal axis thereof soas to induce a magnetic field in the ob- .iect of a direction transverseto the supposed direction of said defects.

.11. The method ofmagnetizing an elongated pose of locating supposeddefects therein, suspected to extend either parallel or transversely tothe longitudinal axis of the object, which consists inenerating;lsolatcdunldirectional electric im-- pulses of high voltageand amperage but individually of minute duration and relatively widelyspaced apart, and passing the impulses through the object diagonally tothe longitudinal axis thereof. p

12. The method of magnetizing a ferro-magnetic object for the purpose'oflocating supposed defects therein, which consists in converting a lowfrequency alternating current into separated unidirectional impulses ofthe same frequency and amplitude as the waves of said current but eachimpulse enduring fora small fraction of the rent, passing said impulsesthrough the object in object of ferro mag'net'ic material for thepurduration of the corresponding wave of said oursuch direction as toinduce a magnetic field therei in of a direction disposed transverselyto the supposed direction of said defects, and simultaneously laterallydisplacing the line of application of the impulses.

13. An apparatus for producing isolatedelectrlcal impulses and passingthe same through an object to induce a magnetic field therein for thedetection of suspected defects, said apparatus comprising'a condenser, acharging circuit therefor, a source of current adapted to energize saidcircuit, a discharge circuit for the condenser including means forsubjecting the object to the influence of the condenserdischarge soas toinduce ;a magnetic field in the object, and control means adapted tolimit discharge of the condenser to isolated impulses at predeterminedintervals.

14. An apparatus for producing isolated electrical impulses and passingthe same through an object to induce a magnetic field therein for the 7detection of suspected defects, said apparatus comprising a condenser, acharging circuit therefor, a source of direct current adapted toenergize said circuit,'a discharge circuit for the condenser includingmeans for subjecting the object to the influence of the condenserdischarge so as to induce amagnetic field in the object, means in saidcircuit permitting discharge of the condenser only at predeterminedperiods, and means for damping oscillations of the condenser discharge.

15. An apparatus for producing isolated electrical impulses and passingthe same through an object to induce a magnetic field therein for thedetection of suspected defects, said apparatus comprising a condenser, acharging circuit therefor, a source of direct current adapted toenergize said circuit, a discharge circuit for the condenser includingcontact terminals adapted to make electrical connection with the objectto pass the condenser discharge therethrough and thereby induce amagnetic field in the object, and control means adapted to limitdischarge of the condenser to isolated impulses at predeterminedintervals.

16. An apparatus for producing isolated electrical impulses and passingthe'same through an object to induce a magnetic field therein for thedetection of suspected defects, said apparatus comprising a condenser, acharging circuit therefor, a source of direct current adapted toenergize said circuit, a discharge circuit for the condenser including apair of terminals adapted to make electrical connection with the objectto pass the condenser discharge through said object, and an electronictube interposed in the discharge circuit and arranged to permitunidirectional discharge of the condenser only at predeterminedintervals.

17. An apparatus for producing isolated electrical impulses and passingthe same through an object to induce a magnetic field therein for thedetection of suspected defects, said apparatus comprising a condenser, acharging circuit therefor, a source of direct current adapted toenergize said circuit, a discharge circuit for the condenser including apair of terminals adapted to make electrical connection with the objectto pass the condenser discharge through said object, and control meansin the discharge circuit arranged to permit unidirectional discharge ofthe condenser only at predetermined intervals, at least one of theterminals being movable along the object in such direction as to spreadthe pulsations of condenser discharge over a broad area.

18. An apparatus for producing isolated electrical impulses and passingthe same through an object to induce a magnetic field therein for thedetection of suspected defects, said apparatus comprising a condenser, acharging circuit therefor, a source of direct current adapted toenergize said circuit, a discharge circuit for the condenser including apair 01' terminals adapted to make electrical connection with the objectto pass the condenser discharge through said object, control meansarranged to limit discharge of the condenser to isolated impulses atpredetermined intervals, a normally open switch in the charging circuitof the condenser, and means adapted to close said switch only when aconnection of predetermined low resistance has been established betweenthe terminals and said object.

19. An apparatus for producing isolated electrical impulses and passingthe same through an object to induce a magnetic field therein for thedetection of suspected defects, said apparatus comprising a condenser, acharging circuit therefor, a source of direct current adapted toenergize said circuit, a discharge circuit for the condenser including apair of terminals adapted to make electrical connection with the objectto pass the condenser discharge through said object, control meansarranged to limit discharge oi the condenser to isolated impulses atpredetermined intervals, a normally open relay switch in said chargingcircuit, a relay circuit for operating said switch, said relay circuitbridging said terminals, and a supply of low voltage current adapted toenergize the relay circuit, said relay circuit being so electricallyproportioned that the relay switch will not close unless a contact oflow resistance has been established between the terminals and theobject.

20. An apparatus for producing isolated electrical impulses and passingthe same through an object to induce a magnetic field therein for thedetection of suspected defects, said apparatus comprising a condenser, acharging circuit therefor, a source of direct current adapted toenergize said circuit, a discharge circuit for the condenser including apair of terminals adapted to make electrical connection with the objectto pass the condenser discharge through said object, control meansarranged to limit discharge of the condenser to isolated impulses atpredetermined intervals, a normally open relay switch in said chargingcircuit, a relay circuit for operating said switch, said relay circuitbridging said terminals, 9. supply of low voltage current adapted toenergize the relay circuit, said relay circuit being so electricallyproportioned that the relay switch will not close unless a contact oflow resistance has been established between the terminals and theobject, and a manually operable switch controlling the relay circuit.

21. An apparatus producing isolated electrical impulses and passing thesame through an object to induce a magnetic field therein for thedetection of suspected defects, said apparatus comprising a condenser, asupply of alternating current, means for rectifying said current andcharging the condenser therewith, a discharge circuit for the condenserincluding a pair of terminals adapted to establish electrical connectionwith said object whereby the condenser will discharge through theobject, an electronic tube interposed in the discharge circuit andarranged to permit unidirectional discharge of the condenser atpredetermined intervals, said tube including a bias grid energized bysaid alternating current and controlling the discharge of the condenserwhereby said discharge will have a frequency corresponding to that ofsaid alternating current.

22. Apparatus for producing isolated electrical impulses of alternatelyopposite polarity and passing the same through an object to induce amagnetic field therein for the detection of suspected defects, saidapparatus comprising a pair of condensers, a charging circuit thereforin which the condensers are connected in parallel, a source of directcurrent adapted to energize said circuit, each condenser having adischarge circuit including an electronic tube adapted to control thedischarge thereof, and a pair of terminals adapted to establishelectrical connection with the object, the discharge circuits of thecondensers being oppositely connected to said terminals whereby thedischarge of one condenser through the object will be in oppositedirection to the discharge of the other condenser therethrough.

23. A tool for applying pulsating current to an elongated object toinduce a magnetic field therein for the detection of suspected defects,said tool comprising a pair of levers pivoted together, a contact brushmounted at the forward extremity of each lever but electricallyinsulated therefrom, a spring acting on the levers to force the brushesinto engagement with opposite sides or said object, and leads connectingsaid brushes to a source of pulsating electric current, one of thebrushes being advanced with respect to the other so that the currentwill pass through the object diagonally with respect to the longitudinalaxis of the object.

24. A method of magnetizing a ferro-magnetic object which consists inelectrically interlinking the object with an electric circuit, producinghigh frequency oscillations of electric current of high voltage andamperage, suppressing all but isolated oscillations at comparativelywidely spaced intervals, and passing said isolated impulses through thecircuit so as to induce a magnetic field in said object.

25. A method of magnetizing a ferro-magnetic object which consists inelectrically interlinking the object with electric circuit, producinghigh frequency oscillations ,of electric current of high voltage andamperage, suppressing all but isolated oscillations at comparativelywidely spaced intervals, passing said isolated impulses through thecircuit so as to induce a magnetic field in said object, and displacingthe object in such direction as to spread the magnetizing effect of theimpulses over a broad area of the object.

26. An apparatus for magnetizing an object, said apparatus comprising acondenser, a charging circuit therefor, a source of current adapted toenergize said circuit, a discharge circuit for the condenser includingmeans for subjecting the object to the influence of the condenserdischarge so as to induce a magnetic field in the object, and controlmeans adapted to limit the discharge of the condenser to isolatedimpulses at predetermined intervals.

27. An apparatus for magnetizing an object, said object comprising acondenser, a charging circuit therefor, a source of direct currentadapted to energize said circuit, a discharge circuit for the condenserincluding means for subjecting the object to the influence of thecondenser discharge so as to induce a magnetic field in the object,means in said circuit permitting discharge of the condenser only atpredetermined periods, and means for damping oscillations of thecondenser discharge.

28. An apparatus for magnetizing an object, said apparatus comprising acondenser, a charging circuit therefor, a source of current adapted toenergize said circuit, a discharge circuit for the condenser including aportion electrically interlinked with the object so as to induce amagnetic field in the object, and control means adapted to limitdischarge of the condenser to isolated impulses at predeterminedintervals.

29. An apparatus for magnetizing an. object, said apparatus comprising acondenser, a charging circuit therefor, a source of current adapted toenergize said circuit, a discharge circuit for the condenser including acoil surrounding said object so that the condenser discharge will inducea magnetic field in the object, means permitting discharge of thecondenser only at predetermined intervals, and means for dampingoscillations of thecondenser discharge.

30. An apparatus for magnetizing an object, said apparatus comprising acondenser, a charging circuit therefor, a source of direct currentadapted to energize said circuit, a discharge circuit for the condenserincluding means for subjecting the object to the influence of thecondenser discharge so as to induce a magnetic field in the object, andan electronic tube interposed in the discharge circuit and arranged topermit uni-directional discharge of the condenser only at predeterminedintervals.

31. The method of locating defects in a ferromagnetic object whichcomprises passing periodic pulsations of magnetic flux through theobject across the supposed defect and in a direc-' make electricalconnection with the object to pass the condenser discharge through saidobject, control means arranged to limit discharge of the condenser toisolated impulses at predetermined intervals, and means adapted to closethe charging circuit of the condenser only when an electrical connectionis established between said terminals.

ALFRED V. nu FORES'I

